tcp: add a scheduling point in established_get_first()

Kubernetes[1] is going to stick with /proc/net/tcp for a while.

This commit reduces the scheduling latency introduced by
established_get_first(), similar to commit acffb584 ("net: diag:
add a scheduling point in inet_diag_dump_icsk()").

In our environment, the scheduling latency affects the performance of
latency-sensitive services like Redis.

Changes in V2 :
 - call cond_resched() before checking if a bucket is empty as
   suggested by Eric Dumazet
 - removed the delay of synchronize_net() from the commit message

[1] https://github.com/google/cadvisor/blob/v0.47.2/container/libcontainer/handler.go#L130Signed-off-by: Jian Wen <wenjian1@xiaomi.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230711032405.3253025-1-wenjian1@xiaomi.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_ipv4.c

@@ -57,6 +57,7 @@
 #include <linux/init.h>
 #include <linux/times.h>
 #include <linux/slab.h>
+#include <linux/sched.h>
 
 #include <net/net_namespace.h>
 #include <net/icmp.h>
@@ -2446,6 +2447,8 @@ static void *established_get_first(struct seq_file *seq)
 		struct hlist_nulls_node *node;
 		spinlock_t *lock = inet_ehash_lockp(hinfo, st->bucket);
 
+		cond_resched();
+
 		/* Lockless fast path for the common case of empty buckets */
 		if (empty_bucket(hinfo, st))
 			continue;